Photolithographic processes are widely utilized in the fabrication of a variety of microstructures. In this regard, a photoresist coating is generally applied to a substrate. The photoresist coating is then illuminated in a pattern defined by a mask positioned between a source of illumination and the photoresist coating. The photoresist coating is then developed and a portion of the photoresist coating, as defined by the illumination pattern, is selectively removed prior to baking the photoresist coating. Thereafter, the substrate may be etched so as to transfer the pattern defined by the photoresist coating to the substrate in order to define a microstructure therein. The remainder of the photoresist coating is then removed to expose the resulting microstructure.
A variety of microstructures can be formed by photolithographic processes. For example, micro-optic devices can be fabricated, such as gratings, micro-lens arrays and the like, as well as micro-fluidic devices.
Some of the microstructures have depth profiles with multi-level plateaus or multi-level channels that desirably have a constant width. Forming such microstructures with a monochromatic mask (including a gray-scale mask) can be problematic since the light with which the photoresist coating is illuminated is absorbed by the photoresist coating in such a manner that the resulting light intensity varies logarithmically with depth within the photoresist coating. As such, neglecting focusing and diffractive effects, the depth profile defined within a photoresist coating by light that has been filtered by a monochromatic mask would also tend to follow a logarithmic contour. Thus, features that are formed deeper within the photoresist coating tend to be narrower than corresponding features that are designed to have the same size that are formed closer to the surface of the photoresist coating. This narrowing of features formed deeper within a photoresist coating is frequently undesirable since the microstructures are oftentimes designed to, instead, have multi-level features of constant width to facilitate optimum operation.
In order to avoid the narrowing of structures formed deeper within a photoresist coating, the microstructure defined by the photoresist coating could be formed in an iterative manner in which a relatively thin photoresist coating was initially exposed and developed. The recesses formed within the initial photoresist coating could then be filled with a removable filler followed by the deposition, exposure and development of a second photoresist coating. This process could continue for as many layers of photoresist coating as desired. However, the process of filling the recess is defined within each photoresist layer is complex and each overlying photoresist layer must be deposited, exposed and developed in a manner that does not disturb the underlying photoresist layers. Still further, the substrate and each photoresist layer must be precisely aligned during each iteration.
Alternatively, the photoresist layer could be exposed and developed to define microstructures at one-depth. The photoresist coating could then be coated with a passivating layer that is then removed from certain portions of the underlying photoresist layer, such as at the higher portions of the photoresist layer, in which additional microstructures will be defined. Those portion of the photoresist layer from which the passivating layer has been removed are then further exposed through another mask so as to define additional microstructures at a different depth. However, this technique also requires multiple coatings and multiple exposures through a sequence of different masks, each of which must be carefully aligned.
As such, it would be desirable to provide a photolithographic process in which three-dimensional microstructures could be more readily formed in photoresist coatings. Moreover, it would be desirable to provide photolithographic processes for efficiently defining three-dimensional structures within a photoresist coating in such a manner that the resulting microstructures have a constant width at various depths within the photoresist layer.